This invention relates generally to zinc plating techniques and more particularly to a method of carrying out zinc plating with the use of a plating bath capable of forming excellent plated surfaces without the use of cyanic compounds.
The zinc electroplating methods practiced heretofore may be divided broadly into alkaline type plating methods in which cyanic compounds are principally used and acidic type plating methods in which zinc chloride, zinc sulfide, and the like are used. Of these, the zinc plating method most widely practiced at present is a strongly alkaline type plating method using a plating bath containing a cyanic compound, particularly a large quantity of sodium cyanide, and a soluble zinc compound.
Since sodium cyanide is contained in large quantity in the plating bath according to this strongly alkaline type plating method, it is possible thereby to obtain a zinc plated surface which is smooth and nearly semibright. It is also known that the finish of this plated surface can be made even better by adding to the plating bath a minute quantity of a brightener by itself or together with other constituents. Examples of suitable brighteners are gelatine, peptone, sodium sulfide, thiourea, polyvinyl alcohol, aldehydes, ketones, and salts of organic acids.
However, since a large quantity of cyanic compounds, which are toxic substances, are used in this alkaline type plating method, the resulting waste liquid cannot be discharged as it is since there is a high risk of this liquid causing pollution. Accordingly, it is necessary to treat the waste liquid by means of expensive equipment, which is not desirable for economical operation. Furthermore, this method gives rise to an undesirable working environment in the plating operation. For example, in the process of zinc plating an iron (steel) base material, iron dissolves in large quantity into the plating bath thereby forming ferrocyanic complex salts or ferricyanic complex salts. Since these complex salts are highly stable, they do not readily decompose into free cyanogen and cannot be completely decomposed and treated by a two-stage treatment by the conventional alkaline chlorine method.
For this reason, alkaline type zinc plating without the use of cyanic compounds is beginning to attract interest, and a typical technique involves the use of a plating bath containing sodium zincate and excess sodium hydroxide is used. When zinc plating is carried out with this plating bath, a spongy zinc without brightness is precipitated, and only a very poor plated surface can be formed. For this reason, efforts are being made to form good plated surfaces by adding brighteners to this plating bath.
Examples of these brighteners are salts of glycollic acid, alkanolamine, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and other alkyleneamines. The use of a brightener of this kind by itself or together with an aromatic aldehyde is also known. However, even by adding such an alkyleneamine brightener to the above mentioned plating bath, it is difficult to form a uniform and homogeneous plated surface. In addition, since the plating conditions for forming a plated surface of good brightness are exact and strict, the utilization of this method is practically impossible for industrial purposes.
Furthermore, it has been found that the product formed from an amine-epoxy reaction is effective as a brightener for an alkaline type plating bath containing a cyanic compound. This brightener comprises principally the reaction product of epichlorohydrin and various amines. For example, in U.S. Pat. No. 2,860,089, a reaction product of epichlorohydrin and ammonia or ethylenediamine is called a polyepoxyamine and is described. Furthermore, a reaction product of epichlorohydrin and hexamine is disclosed in U.S. Pat. No. 3,227,683.
These brighteners exhibit considerable effectiveness in plating baths contain cyanic compounds since the epoxy ring reacts with water, whereby two OH radicals are formed, and, moreover, the chlorine of the epichlorohydrin reacts with NH.sub.2 to become a quaternary ammonium salt. Attempts have been made to improve brightness by adding brightness of this character into plating baths which do not contain cyanic compounds.
However, since the plating film hardness is high, peeling off of the plated surface frequently occurs during working of the plated material, whereby this improvement of brightness cannot be said to be satisfactory, and, moreover, the rate of plating film deposition is slow. For this reason, while this practice is possible on a laboratory scale, it is not suitable for quantity (mass) production on an industrial scale. Furthermore, in comparison with the case of plating with the use of a plating bath containing a cyanic compound, it is more difficult to form a uniform surface, and, moreover, it is difficult to obtain a bright finish on a plated surface electrodeposited by a low current. These adverse features of this practice indicate that it is not suitable for a rack type plating operation.